System for balancing the voltage of series-connected semiconductor elements

ABSTRACT

A circuit for balancing a voltage across a semiconductor element series-connected with other semiconductor elements of the same type may include a comparator configured to compare data representative of a voltage across the semiconductor element with a reference voltage, and a resistive element of adjustable value and configured to be controlled by the comparator.

RELATED APPLICATION

This application claims the priority benefit of French Patent application number 13/61454, filed on Nov. 21, 2013, the contents of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present disclosure generally relates to electronic circuits and, more generally, to a voltage-balancing system intended for series-connected semiconductor devices. The present disclosure more specifically relates to power applications.

BACKGROUND

Frequently, a plurality of semiconductor components, to which are applied, together, a relatively high voltage with respect to the individual voltages that the different components have to withstand, are series-connected. Such is in particular the case for rectifying diodes series-connected between two terminals where a high voltage is applied (typically, several hundreds of volts). Each diode then sees a voltage lower than the total voltage. Ideally, the voltage seen by each diode, or more generally by each semiconductor element, corresponds to the total voltage divided by the number of components. In practice, due to technological dispersions in the forming of semiconductor devices, to manufacturing tolerances, and dispersions due to a temperature difference between each element in the application, there frequently is a voltage imbalance between the different diodes. This may lead to a situation where one of the diodes sees, between its terminals, a voltage which exceeds the voltage that it can withstand.

Systems that include a protection resistor connected in parallel with each diode to achieve a static balance and a capacitor connected for a dynamic protection have already been provided. However, devices based on resistors in parallel with the diodes have the major disadvantage of generating permanent losses through the resistors.

SUMMARY

An embodiment of the present disclosure overcomes all or part of the disadvantages of usual balancing systems. Another embodiment provides an approach for decreasing losses with respect to a system where balancing resistors in parallel with each element are used.

Another embodiment provides a self-adaptive approach. Another embodiment provides an approach that does not use a control signal from the outside.

Thus, an embodiment provides a circuit for balancing a voltage across a semiconductor element series-connected with other semiconductor elements of the same type. The circuit includes a comparator of data representative of the voltage between the terminals of the semiconductor element with a reference voltage. The circuit also includes an adjustable value resistive element that is controlled by the comparator.

According to an embodiment, the reference voltage is representative of the voltage applied to the series association of the semiconductor elements, divided by the number of elements. According to an embodiment, the resistive element is a metal oxide semiconductor (MOS) transistor.

According to an embodiment, the comparator is a first operational amplifier having a first input receiving the data representative of the voltage between the terminals of the semiconductor element and having a second input receiving the reference voltage. According to an embodiment, the reference voltage is provided by a second operational amplifier assembled as a subtractor.

According to an embodiment, the data representative of the voltage across the semiconductor element is sampled from the midpoint of a first resistive dividing bridge connected between the terminals of the semiconductor element. According to an embodiment, the circuit comprises two first terminals intended to be connected between the terminals of said semiconductor element, and two second terminals to be respectively connected to a previous circuit and to a next circuit in the series association of the semiconductor elements.

According to an embodiment, a second resistive dividing bridge connects the two second terminals. The midpoint of the second dividing bridge is connected to a first input of the second operational amplifier having a second input connected, by a second resistive element, to the other second terminal, and by a third resistive element to one of the first terminals.

A system for self-balancing series-connected semiconductor elements includes a circuit such as described above connected in parallel on each semiconductor element. According to an embodiment, the semiconductor elements are diodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic diagram of a series association of semiconductor elements to which embodiments of the present invention apply.

FIG. 2 is a schematic block diagram of an embodiment of a system for balancing the voltages across series-connected semiconductor elements according to the present invention.

FIG. 3 is a more detailed schematic diagram of a balancing circuit of the system of FIG. 2.

FIG. 4 is a schematic diagram of an interconnection of circuits in FIG. 3.

DETAILED DESCRIPTION

The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. The same elements have been designated with the same reference numerals in the different drawings. For clarity, only those steps and elements which are useful to the understanding of the embodiments which will be described have been shown and will be detailed. In particular, the destination of a series association of semiconductor elements has not been detailed, as the described embodiments are compatible with usual applications that use a plurality of series-connected semiconductor devices.

FIG. 1 is a simplified representation of an example of a series association of elements or components D₁, D₂, . . . , D_(n-1), D_(n) having a voltage V_(T) applied between end terminals 12, 14 of their series association. Each device sees, between its terminals, a voltage V_(Di) (i being in the range from 1 to n, and n standing for the number of series-connected components).

The most current applications of such series associations of semiconductor components relate to protection devices, for example, AC/DC or DC/DC voltage conversion devices such as high-voltage switched-mode power supplies capable of being used in fields such as traction, solar inverters, laser control, etc. Thus, devices D₁ to D_(n,) for example, are semiconductor diodes but may more generally be any semiconductor component, for example, MOS transistors, insulated-gate bipolar transistors (IGBTs), switches, Schottky diodes, zener diodes, etc.

In an assembly of the type in FIG. 1, technological dispersions between the different components may result in that one or a plurality thereof sees between their terminals a voltage which exceeds the voltage that they can withstand. Taking the example of semiconductor diodes, these are given for a repetitive reverse maximum voltage (V_(RRM)) and a maximum reverse current (I_(R)) corresponding to the maximum instantaneous value which, for voltage V_(RRM), corresponds to the maximum junction temperature. A maximum reverse voltage (V_(RM)), which corresponds to value V_(RRM) multiplied by a safety factor k, is then defined.

In known circuits, a resistor (not shown in FIG. 1) is connected in parallel with each diode to statistically balance the voltage, and capacitances are generally also assembled, individually and in parallel on each diode, for dynamic balancing purposes. The resistors and capacitances are sized so that the voltage across each diode of the series association does not exceed value V_(RN). However, a problem of permanent resistive loss on operation of the circuit arises. Further, the resistors generally have to be sized to take the worst case into account. This results in increasing losses in normal operation.

FIG. 2 is a simplified representation of an embodiment of a system where an active circuit C_(i) (i being in the range from 1 to n) is connected in parallel with each component, in this example, diodes D_(i) (D₁ to D_(n)). Each circuit C_(i) is connected to terminals K_(i) and A_(i) of the concerned diode. Since the diodes are in series, anode A_(i) of a diode of rank i is common (or connected) to cathode K_(i+1), of the diode of rank i+1. In the example of direction taken in the drawings, the cathode of diode D₁ defines terminal 12 of the application of the high potential of voltage V_(T) and anode A_(n) of diode D_(n) defines terminal 14 of the application of the low potential of voltage V_(T).

Each circuit C_(i) is connected to circuits C_(i-1) and C_(i+1) which are adjacent thereto by its connection to the anode and cathode of the diode associated therewith (except for the first C₁ and last C_(n) circuits which are only respectively connected to the next circuit C₂ and to the previous circuit C_(n-1)). Further, each circuit C_(i) is also directly connected to its adjacent circuits C_(i-1) and C_(i+1), with the same exception for the first and last circuits, independently from the connection via its diode. This will better appear from the following description of FIG. 3.

FIG. 3 shows the detailed electric diagram of an embodiment of a circuit C_(i). FIG. 4 illustrates the connection (or assembly) of n circuits in a series association of n diodes.

Each circuit C_(i) comprises two terminals 21 and 23 to be connected to respective electrodes K_(i) and A_(i) of the concerned diode, and two terminals 25 and 27 to be respectively connected to circuits C_(i-1) and C_(i+1). Circuit C₁ has its terminal 25 connected to its terminal 21 (and thus to terminal 12). Circuit C_(n) has its terminal 27 connected to terminal 23 (and thus to terminal 14).

Although, in the following description of circuit C_(i), the case of resistive elements made in the form of resistors will be taken as an example as the “resistors” may take different forms (for example, the form of MOS transistors). Indeed, the values of the resistive elements of circuit C_(i) are compatible with an embodiment in the form of an integrated circuit.

A resistive dividing bridge formed by two resistors R1 and R2 series-connected between terminals 21 and 23 provides data V_(FBi) relative to the voltage across diode D_(i) (not shown in FIG. 3). Data V_(FBi) sampled from midpoint 22 of this series association is sent onto a non-inverting terminal (+) of an operational amplifier 31 assembled as an analog comparator with a reference level V_(REF). The output of amplifier 31 is connected to a control electrode of a transistor, for example, a MOS transistor M (for example, of type N). Transistor M operates in a linear mode, that is, its gate control signal varies its on-state series resistance (RdsON). Transistor M connects terminals 21 and 23. Comparator 31 is assembled as an inverter as its output is connected to its inverting input terminal (−) by a resistor R3, and reference level V_(REF) is applied via a resistor R4, preferably of same value as resistor R3. The values of resistors R1 and R2 are selected according to the supply voltage of the comparator 31 and to the maximum voltage between terminals 21 and 23 (maximum voltage V_(Di) capable of being seen by diode D_(i)).

Assuming that the diodes have identical characteristics (same voltage V_(RRM)), the resistances R1 of all circuits C_(i) preferably have the same value, and the resistances R2 of all the circuits preferably have the same value. The same is true, on the one hand, for resistances R3, and on the other hand, for resistances R4.

Reference level V_(REF) is generated by an operational amplifier 33 assembled as a subtractor and having the function of dividing by number n of diodes D_(i) of the series association voltage V_(T) present between terminals 12 and 14, and for each diode D_(i), referencing this value to its anode (terminal 23 of circuit C_(i)). The non-inverting input (+) of the amplifier 33 is connected to its output 24 by a resistor R5 as well as, by a resistor R6, to junction point 26 of resistors R7 and R8, series-connected between terminals 25 and 27 of circuit C_(i). The inverting input (−) of amplifier 33 is connected, by a resistor R9, to terminal 27 and, by a resistor R10, to terminal 23, to reference voltage V_(REF) to anode A_(i) of diode D_(i).

Each circuit C_(i) is connected to the adjacent circuits by its terminals 25 and 27, terminal 25 of circuit C_(i) being connected to terminal 27 of circuit C_(i-1) of lower rank and terminal 27 of circuit C_(i) being connected to terminal 25 of circuit C_(i+1) of higher rank. Terminal 25 of circuit C₁ is connected to terminal 12 and terminal 27 of circuit C_(n) is connected to terminal 14.

Assuming that the diodes have identical characteristics (same voltage V_(RRM)), the resistances R7 of all circuits preferably have the same value, and the resistances R8 of all the circuits preferably have the same value. These values are selected according to the power supply voltage of amplifiers 33 and to the maximum values capable of being taken by voltages V_(RRM) of the diodes and V_(T). Resistances R1 and R7 have the same value and resistances R2 and R8 have the same value.

Further, resistances R5 preferably have the same value, resistances R6 preferably have the same value, resistances R9 preferably have the same value, and resistances R10 preferably have the same value. According to a specific embodiment, the values of resistances R5, R6, R9, and R10 are all identical. Resistance R3 is preferably much higher than resistance R4 (for example, by a factor between 10 and 100) to have a high gain of the system in closed loop.

Circuit C_(i) is in charge of measuring the voltage across diode D_(i) and of comparing this voltage to total voltage V_(T) divided by the number of diodes, to deviate, if desired, part of the current through transistor M, and thus balance the different voltages V_(Di) of the series association. The system control results in having voltage V_(FBi) tending, for a balanced system, towards voltage V_(REF).

In case of an imbalance, voltage V_(FBi) is equal to a given ratio of voltage V_(Di) while V_(REF) is equal to a given ratio of voltage V_(T.) This ratio is for example equal to R/(R+R′), where R stands for the value of resistances R1 and R7, and where R′ stands for the value of resistances R2 and R8. Current I_(revi) in each diode D_(i), added to current I_(outi) in transistor M of circuit C_(i) associated therewith, is identical for each diode D_(i)/balancing circuit C_(i) association. The gain of amplifier 31 and the ratio between current I_(outi) (and thus resistance R_(dsON)) and gate-source voltage V_(gs) of transistor M is selected according to the value of total voltage V_(T) and to number n of diodes.

An advantage of the described embodiments is that the power consumption in transistor M of the balancing circuit only occurs in the presence of an imbalance. Indeed, the power consumption due to resistive elements R1 and R10 and to amplifiers 31 and 32 is negligible since the currents are relatively low (typically less than at least 100 times the dissipation for transistor M).

The described embodiments use a power source for amplifiers 31 and 33. The power supply may be extracted from the voltage across the concerned diodes or be sampled from another voltage source of the system.

Various embodiments have been described. Various alterations, modifications, and improvements will occur to those skilled in the art. For example, in the case of diodes having nominal characteristics different from one another, the values of the different resistances should be adapted. Further, although reference has been made to a MOS transistor M to deviate the current, the element may more generally be any controllable resistive element, that is, having an adjustable resistance value, for example, a bipolar transistor. Further, the practical implementation of the described embodiments, in particular the sizing of the components of circuits C_(i), is within the abilities of those skilled in the art based on the functional indications described above.

Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto. 

1-10. (canceled)
 11. A circuit for balancing a voltage across a semiconductor element series-connected with other semiconductor elements of a same type, the semiconductor elements having terminals, the circuit comprising: a comparator configured to compare data representative of a voltage between the terminals with respect to a reference voltage; and a resistive element having an adjustable value coupled to said comparator and configured to be controlled by said comparator.
 12. The circuit of claim 11, wherein the reference voltage is representative of a voltage applied to the series-connected semiconductor element and other semiconductor elements divided by a number of semiconductor elements.
 13. The circuit of claim 11, wherein said resistive element comprises a metal oxide semiconductor (MOS) transistor.
 14. The circuit of claim 11, wherein said comparator comprises a first operational amplifier having a first input receiving the data representative of the voltage across the semiconductor element and having a second input receiving the reference voltage.
 15. The circuit of claim 14, further comprising a second operational amplifier coupled to the first operational amplifier and configured as a subtractor and to generate the reference voltage.
 16. The circuit of claim 14, further comprising a first resistive dividing bridge having a midpoint and coupled between the terminals of said semiconductor element; and wherein the data representative of the voltage between the terminals of said semiconductor element is sampled from the midpoint.
 17. The circuit of claim 11, further comprising: first terminals configured to be connected between the terminals of said semiconductor element; and second terminals configured to be respectively connected to a previous circuit and to a next circuit in the series-connected semiconductor element and other semiconductor elements.
 18. The circuit of claim 15 further comprising: a first resistive dividing bridge having a midpoint and coupled between the terminals of said semiconductor element, and wherein the data representative of the voltage between the terminals of said semiconductor element is sampled from the midpoint; first terminals configured to be connected between the terminals of said semiconductor element; second terminals configured to be respectively connected to a previous circuit and to a next circuit in the series-connected semiconductor element and other semiconductor elements a second resistive element; a third resistive element; and a second resistive bridge coupled to said second terminals and having a midpoint, wherein said second operational amplifier has first and second inputs, and wherein the midpoint of the second resistive bridge is coupled to the first input of said second operational amplifier, wherein said second input of said second operational amplifier is coupled, by said second resistive element, to one of said second terminals, and, by said third resistive element, to one of said first terminals.
 19. A system comprising: a plurality of series-connected semiconductor elements; a circuit coupled in parallel on each of said plurality of series-connected semiconductor elements, said circuit comprising a comparator configured to compare data representative of a voltage between the terminals with respect to a reference voltage, and a resistive element having an adjustable value coupled to said comparator and configured to be controlled by said comparator.
 20. The system of claim 19, wherein said plurality of series-coupled semiconductor elements comprises diodes.
 21. A method of balancing a voltage across a semiconductor element series-connected with other semiconductor elements of a same type, the semiconductor elements having terminals, the method comprising: using a comparator to compare data representative of a voltage between the terminals with respect to a reference voltage; and controlling a value of a resistive element having an adjustable value based upon the comparator.
 22. The method of claim 21, wherein the reference voltage is representative of a voltage applied to the series-connected semiconductor element and other semiconductor elements divided by a number of semiconductor elements.
 23. The method of claim 21, wherein said resistive element comprises a metal oxide semiconductor (MOS) transistor.
 24. The method of claim 21, wherein using the comparator comprises using a first operational amplifier having a first input receiving the data representative of the voltage across the semiconductor element and having a second input receiving the reference voltage.
 25. The method of claim 24, further comprising using a second operational amplifier coupled to the first operational amplifier as a subtractor to generate the reference voltage. 